The present invention relates to a method for optimizing sootblowing in steam boilers. It is particularly well adapted for use with chemical recovery boilers used with the kraft pulping process.
The combustion of most fuels results in large quantities of condensible gases and particulates carried up through the boiler with the hot flue gas. In some cases these particulate materials deposit on the boiler tubes where they act as an insulating layer which reduces the thermal efficiency of the boiler. In addition, the ash buildup restricts the gas flow through the tube banks limiting the output of the boiler and sometimes forcing shutdowns. In the case of kraft black liquor combustion, the amount of ash deposited is very large and its removal on a continuing basis is critical to the operation of the boiler.
To minimize the impact of continuous ash buildup in boilers where this occurs, devices using high pressure air or steam called sootblowers are commonly used. The sootblowers are typically in the form of long tubes or conduits which slowly advance into and then withdraw from the boilers and are timed to be blown sequentially over and over again. Steam usage can be very significant, amounting to up to 12% or even more of the total steam generated under certain load conditions. Traditionally, sootblowers have been timed to operate on a fixed cycle regardless of the actual condition of the tubes.
Within the past decade, various attempts have been made to reduce the amount of sootblowing steam. The more sophisticated of these systems attempt to determine the fouling rates for various sections of the boiler. With this information, it is believed that one can make intelligent decisions as to when and where cleaning should be done. These systems are normally computerized and are programmed to perform the calculations required to determine the fouling rate of each portion of the boiler and to select the zones to be cleaned by the sootblowers. This approach may involve the use of instrumentation within the boiler to determine gas side pressure and temperature drops and flow rates through given portions of the tube bank. It may also include instrumentation to measure waterside flow rates and temperature changes. One such system is described by Pantsar, Pulp & Paper, 53(9): 142-145 (1979). This system, in addition to using the heat transfer criteria previously noted, also sets a maximum time interval for blowing the various sections of the boiler. Apparently these sections are treated as independent units so that the sections having higher fouling rates are blown more frequently than those with lower rates of deposit buildup.
A paper by Carter and Mathieson, Pulp and Paper Canada, 82(2): 84-88 (1981), describes a system similar in many respects to that described by Pantsar. These authors also stress the need to measure heat transfer characteristics in various sections of the boiler. In addition, they note that all of these measurements are dependent upon the firing rate. To compensate for this, all differential temperature or pressure measurements are normalized for boiler load. Minimum time limits are set for each section to avoid overblowing from measurement aberrations. Likewise, maximum time limits are set to ensure that a minimum amount of cleaning is done regardless of the information contributed by the instrumentation. Each section of the boiler is assigned a priority so that the most critical section is chosen if more than one is due for blowing.
U.S. Pat. No. 4,454,840 to Dziubakowski relates to an optimized scheduled timing of sootblowing in which the scheduling is set empirically. The method is based on the use of a relative boiler efficiency measurement. The inventor does recognize an interdependence between various sections or "heat traps" within the boiler. One manifestation of this is seen where two units are scheduled for blowing at approximately the same time. In this case, priority is given to the upstream unit to avoid refouling a clean downstream unit. Other than this situation, units are blown when scheduled without regard to their position in the boiler.
A paper by Pelletier and Gettle, Pulp & Paper, 53(2): 127-129 (1979), suggests that the best way of telling when blowing is needed in a section is visual observation. This can be supplemented by operating parameters such as exit gas temperature, steam efficiency, and draft losses. The authors note that rather than continuous blowing, some sections of the boiler needed only to be blown once a shift or once a day.
A paper by Mason et al presented at the TAPPI 1977 Engineering Conference notes that many modern boilers have sootblowers arranged and grouped according to the type of heating surface. This enables one group of sootblowers to be disabled if cleaning is not required in that particular area. They also note that it may be possible to program several blower sequences into a panel to coordinate blower operation with boiler load. The authors also note the cost savings that can be effected in steam usage by building sootblower idle time where this is possible.
A paper by Hoynalanmaa, Pulp & Paper, 54(8): 97-99 (1980), describes feed forward disturbance compensation in a sootblowing strategy. This strategy is based on direct measurements of critical parameters and on values calculated from these measurements. All measurements are normalized for flue gas flow. Sootblowing is carried out locally and only when deemed necessary.
At least three computerized sootblowing control system are commercially available at present in the United States. All of these appear to be derivatives of one of the systems just discussed. One system, available from Fisher Control, Marshalltown, Iowa, schedules sootblowing as a function of boiler solids load. Exit gas temperatures from the boiler, economizer, and stack, as well as pressure drops across various sections, are continuously monitored. Sootblowing is done more frequently in areas which tend to foul worst. The boiler is divided into four zones which encompass the economizer, boiler bank, superheater front, and superheater rear and intermediate tube groups. There is no apparent overall integration between these zones except that only one may be blown at a time.
Another computer controlled sootblowing system is available from the Bailey Controls Division of Babcock & Wilcox, a McDermott Company. This appears to be based on the system described in the Dziubakowski patent. Boiler efficiency is calculated on line using a computer based model, and optimum blowing cycle time is calculated for each heat absorbing unit. The model is updated as boiler operating conditions change. Changes which are due to transducer "noise" are minimized by averaging readings over time. The system employes maximum and minimum cycle times and senses maximum allowable pressure drops across tube groups. A primary consideration appears to be minimizing the amount of sootblowing steam required by the boiler.
A third system is available from Measurex Systems, Inc., Cupertino, Calif. This system has many commonalities with the two just described. The boiler is divided into several sections, each of which has instrumentation indicating the state of fouling. In the event of more than one section being scheduled to blow at a particular time, the software will assign priority. All of the sootblowers in any unit are not necessarily blown each time. Various fixed combinations have been defined which can be chosen depending upon the immediate conditions.
Minimizing sootblowing steam consumption appears to be the primary goal of all the systems just described. The present inventors have taken a different approach and have as primary goals the prevention of boiler plugging and maintenance of boiler efficiency at the highest practical level. Minimization of sootblowing steam is a secondary consideration.